1. Field of the Invention
This invention relates to improved waterflooding operation in subterranean reservoirs involving the injection of molecular oxygen-containing water into the reservoir.
2. Description of the Prior Art
In the recovery of oil form oil-bearing subterranean reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces in the reservoir. In order to increase the production of oil from subterranean reservoirs, resort has been taken to a variety of supplemental (secondary) recovery techniques. The most widely used supplemental technique is waterflooding, which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein toward a production system comprising one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus, it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
The effects of reservoir temperature on oil viscosity are related to the oil's viscosity at a reference temperature and the oil's paraffinicity. A viscosity/temperature plot for a wide variety of crude oils is shown in W. B. Braden, Annual Fall Meeting of Soc. Pet. Eng., SPE No. 1580, Oct. 1966, which is incorporated herein by reference. Water's (brine) viscosity changes less rapidly with temperature. Water viscosities will all lie below the ranges in the plot.
The purpose of this invention is to supplement a waterflood operation by reducing the viscosity of the oil being produced, thereby increasing flowability and improving sweep efficiency. This is accomplished by dissolving molecular oxygen gas under pressure in the injected water, whereby controlled oxidation of part of the crude oil trapped in the porous rock liberated heat that reduces the viscosity of the remaining oil, thus stimulating its displacement and recovery. Insofar as is now known, this operation has not been proposed.